A new species of Platypodiella Guinot, 1967, from the island of St. Helena, P. georgei spec, nov., is described. This species was found associated with encrusting species of Palythoa Lamouroux, 1816 (Zoantharia). Ample evidence is presented that both the eastern Atlantic P. picta (A. Milne-Edwards, 1869) and the western Atlantic P. spectabilis (Herbst, 1794) also live habitually associated with encrusting species of Palythoa, rarely with other zoantharians. For each of the two East-Pacific members of the genus, P. rotundata (Stimpson, 1860) and P. gemmata (Rathbun, 1902), there is at least one rather detailed published record suggesting that these species too associate with Palythoa. Thus, this habit would seem to go for all five members of the genus. Data are added on two other crab species, viz.Pitho Iherminieri (Desbonne & Schramm, 1867) (Majidae) (western Atlantic) and Banareia armata A.Milne Edwards, 1869 (Xanthidae) (Indonesian waters), which may also associate with Palythoa colonies.